1. Field
This disclosure relates generally to LDMOS, and more specifically, to LDMOS with soft operating area.
2. Related Art
Laterally double diffused MOS (LDMOS) has been very effective for providing control in high power applications. One issue that developed as the ability to dissipate power increased was issues with safe operating area. At operating conditions in which there is both high current and high voltage, impact ionization is high enough to significantly increase the body current, typically P type, so that it forward biases the source-body junction. This particular PN junction was also the base emitter of a parasitic NPN bipolar transistor formed between the extended drain region, the body and source of the LDMOS. When this parasitic transistor became conductive, it entered into a positive feedback mechanism where this current pickup further increased impact ionization causing the parasitic bipolar to become even more conductive. In a matter of milliseconds, the device would be destroyed. A device topology called double RESURF was developed to spread the drain current so that not all of the electrons that crossed the channel to the drain exited at the drain contact. A significant portion of the electrons exited through a buried layer under the drain. This significantly reduced the amount of impact ionization caused as a result of current crowding and thus increased the voltage that the device could pass before reaching the situation where the parasitic bipolar was triggered and would destroy the device. The effectiveness, however, was better if the buried layer was relatively shallow. The relatively shallow buried layer, however, decreased the voltage breakdown between the buried layer and the body which limited the overall blocking capability of the device. Thus, there was a tradeoff between the maximum achievable breakdown of the double RESURF structure and its effectiveness in improving the safe operating area of this device.
Accordingly, there is a need to retain effective control of impact ionization in order to achieve a desirable safe operating area while increasing the breakdown voltage between the buried layer and the body.